The present invention relates to a pixel circuit for current-driving a light-emitting element provided for each pixel. The invention also relates to a display that includes the pixel circuits arranged in a matrix (in rows and columns), and particularly to an active-matrix display that employs insulated-gate field effect transistors provided in the respective pixel circuits and controlling the amount of a current applied to a light-emitting element, such as an organic electro-luminescence (EL) element.
In an image display, e.g., in a liquid crystal display, a number of liquid crystal pixels are arranged in a matrix, and the transmittance intensity or reflection intensity of incident light is controlled on each pixel basis in accordance with information of images to be displayed, to thereby display the images. A similar principle also holds for an organic EL display employing organic EL elements for pixels. The organic EL element however is a self-luminous element unlike the liquid crystal pixel. Therefore, the organic EL display has advantages over the liquid crystal display: high image visibility, no backlight, and high response speed. Furthermore, the organic EL display is a current-control display, which allows control of the luminance (gray-scale) of each light-emitting element by a current applied to the emitting element, and therefore is significantly different from a liquid crystal display, which is a voltage-control display.
Driving systems for the organic EL display include a simple-matrix system and an active-matrix system similarly to the liquid crystal display. The simple-matrix system employs a simple configuration, but involves difficulties of fabricating large-size and high-definition displays. Therefore, the active-matrix displays have been developed more actively in recent years. In the active-matrix system, a current applied to a light-emitting element in each pixel circuit is controlled by an active element (typically a thin film transistor (TFT)) provided in the pixel circuit. Examples of the active-matrix system have been disclosed in Japanese Patent Laid-opens No. 2003-255856, 2003-271095, 2004-133240, 2004-029791, and 2004-093682.
A pixel circuit in related art is disposed at each of intersections between row scan lines that supply control signals and column signal lines that supply video signals. Each pixel circuit includes at least a sampling transistor, a capacitive part, a drive transistor and a light-emitting element. The sampling transistor conducts in response to the control signal supplied from the scan line, to sample the video signal supplied from the signal line. The capacitive part holds an input voltage corresponding to the sampled video signal. The drive transistor supplies an output current during a certain emission period depending on the input voltage held by the capacitive part. Typically the output current has dependence on the carrier mobility in the channel region of the drive transistor and the threshold voltage of the drive transistor. The output current supplied from the drive transistor causes the light-emitting element to emit light with a luminance dependent upon the video signal.
The drive transistor receives at the gate thereof the input voltage held by the capacitive part, and conducts the output current between the source and drain thereof, to thereby apply the current to the light-emitting element. Typically the emission luminance of the light-emitting element is proportional to the applied current amount. In addition, the amount of the output current supplied from the drive transistor is controlled by the gate voltage, i.e., the input voltage written to the capacitive part. The pixel circuit in the past changes the input voltage applied to the gate of the drive transistor depending on the input video signal, to thereby control the amount of a current supplied to the light-emitting element.
The operating characteristic of the drive transistor is expressed by Equation 1.Ids=(½)μ(W/L)Cox(Vgs−Vth)2  Equation 1
In Equation 1, which is a transistor characteristic equation, Ids denotes a drain current flowing between the source and drain. This current is an output current supplied to the light-emitting element in the pixel circuit. Vgs denotes a gate voltage applied to the gate based on the potential at the source. The gate voltage is the above-described input voltage in the pixel circuit. Vth denotes the threshold voltage of the transistor. μ denotes the mobility in a semiconductor thin film serving as the channel of the transistor. In addition, W, L and Cox denote the channel width, channel length and gate capacitance, respectively. As is apparent from Equation 1, when a thin film transistor operates in its saturation region, the transistor is turned on to conduct the drain current Ids if the gate voltage Vgs is larger than the threshold voltage Vth. In principle, a constant gate voltage Vgs invariably supplies the same drain current Ids to the light-emitting element as shown by Equation 1. Therefore, supplying video signals having the same level to all pixels in a screen should allow all the pixels to emit light with the same luminance, and thus should achieve uniformity of the screen.
In fact, however, thin film transistors (TFT) formed of a semiconductor thin film, such as a poly silicon film, involve variation in the device characteristics. In particular, the threshold voltage Vth is not constant but varies from pixel to pixel. As is apparent from Equation 1, even if the gate voltage Vgs is constant, variation in the threshold voltage Vth among the drive transistors leads to variation in the drain current Ids. Thus, the luminance varies depending on each pixel, which spoils uniformity of the screen. In related art, there has been developed a pixel circuit that has a function of canceling variation in the threshold voltage among drive transistors. For example, this pixel circuit is disclosed in the above-mentioned Japanese Patent Laid-open No. 2004-133240.
The pixel circuit provided with the function of canceling variation in the threshold voltage can improve uniformity of a screen to some extent. However, of the characteristics of poly-silicon TFTs, not only the threshold voltage but also the mobility μ vary depending on each element. As Equation 1 shows, variation in the mobility μ results in variation in the drain current Ids even if the gate voltage Vgs is constant. As a result, emission luminance varies from pixel to pixel, which problematically spoils uniformity of a screen.